There is an increasing demand for lightweight electronic devices mounted on flying bodies such as aircrafts, artificial satellites and missiles. In recent years in particular, with the increased kinds and numbers of electronic apparatuses mounted on such flying bodies, the requirements on lightweight housings for housing electronic parts incorporated in electronic apparatuses have become even more demanding.
Aluminum has been conventionally used as a material for a housing of this type. Aluminum is light in weight (specific gravity if about 2.7 g/cc) but has a large thermal expansion coefficient (24.times.10.sup.-6 /.degree.C.) Therefore, in connecting a ceramic substrate having a thermal expansion coefficient of 6-7.times.10.sup.-6 /.degree.C., with electronic parts mounted thereon to a housing by soldering using a lead tin, gold tin or the like solder, the difference between the thermal expansion coefficient of the substrate and that of the housing is so large that warping in a junction portion and cracking in the ceramic substrate are produced.
In addition, an increased number of ceramic substrates has been recently used for mounting FETs or ICs requiring a large heat dissipation. In this case, the ceramic substrate is fixed to a housing by soldering or the like and the housing is cooled, thereby causing the generated heat to escape through the cooled housing to the exterior. In such a case, since aluminum having a large thermal expansion coefficient cannot be used as a material for the housing, such materials as Fe-Ni-Co with a specific gravity of, of 10-11 g/cc, and a thermal conductivity of 0.03 cal/cm.sec. .degree.C. or similar materials are used, sacrificing lightness and a thermal conductivity.
It is also known to use an Al-Si compound material manufactured by a rapid solidification method, as a material for a substrate on which a semiconductor device is mounted and as a material for a heat radiation fin, see U.S. Pat. Nos. 4,830,820, 4,926,242. The Al-Si compound material comprises 35-40% by weight of Si: 65-55% by weight of Al is manufactured by the quenching method and has a specific gravity of about 2.5 g/cc which is smaller than that of aluminum, and a relatively high thermal conductivity of 0.3 cal/cm.sec.multidot..degree.C. Therefore, such material is considered to be suitable for making housings of the type mentioned.
A housing formed of this Al-Si compound material is made by hot working and cutting, alloy powder obtained by rapid solidification melted Al 60% by weight and Si 40% by weight through an atomizing method.
However, this Al-Si compound material has the following disadvantages when used for a housing.
First, distortion is produced when processing such a housing. That is, since a housing of this kind in general should be cooled when used, the housing must be attached to a cooling device. The deformation of the rear of the housing should be accordingly minimized by correcting the same by a heat treatment. Heating the housing obtained by such methods as described above, that is, obtained by cutting a material made of quenched powder formed by hot working, causes the housing to be greatly distorted due to an occlusion gas (N.sub.2, H.sub.2, H.sub.2 O etc.) included in the material. In this case, even if the heating is carried out in a vacuum or in an inactive gas such as argon, the above-described distortion is produced due to the emission of the occlusion gas. This is the first disadvantage.
A second disadvantage is the thermal distortion after plating. In many cases, a surface of such a housing should be plated with Ni, Ni+Au or the like to make it easier to solder the housing and to give the same a good corrosion resistance. The heating performed after the plating results in emission of an occlusion gas which cause distortion of the housing. The heating after soldering the ceramic substrate or the like to the housing also results in emission of an occlusion gas, thereby further distorting the housing. In this case, the plated layer is expanded. The foregoing is the second disadvantage.
A third disadvantage is seen in that welding a cover made of an aluminum alloy, to the housing by using a laser beam, produces bubbles at the welded portion and causes the formation of coarse Si particles. Both of these phenomena significantly deteriorate the airtightness and the hardness reliability after the welding of the cover to the housing. While the bubbles are mainly caused by the emission of an occlusion gas as described in the first disadvantage at the time of fusing a welded portion, these bubbles are also caused the coarse Si particles which will be described in the following. The Al-Si compound material formed by the quenching method is characterized by the dispersion of fine particles of Si into the Al through rapid solidification, and in that, when the melted portion at the time of welding is slowly cooled, the Si particles become coarse when being solidified, thereby producing cavities in the Al in the periphery of the Si particles, which makes the welded portion fragile.